A multi-phase or “interleaved” electrical circuit performs a function (normally done by a single electrical component or phase) using two or more parallel, duplicate components or phases that are activated at different, sequential points in time. A control or clock signal is divided into time slices and enables a different phase during each time slice; the other phase or phases are disabled. Because the devices of each phase are identical or nearly identical, the output of the interleaved circuit is the same as if it were a single, always-on circuit. For example, a first phase of an interleaved circuit may be enabled when a clock signal is high, and a second phase may be enabled when the clock signal is low; each phase receives the same input and contributes to the same output.
While interleaving may be used in any circuit, a major application of interleaving is in power systems. Interleaved phases reduce the stress that any one individual component experiences (because the component may be active only 50% or less of the time), thereby extending the lifetime and reliability of high-current power circuits. Other circuits, such as AC-to-DC power-factor-correction (“PFC”) circuits, which are used to improve the efficiency of power delivered to electronic components, may receive additional benefits from interleaved phases. For example, the increased switching frequency introduced by interleaving may reduce the PFC circuit's input current ripple (thereby simplifying the design of an upstream electromagnetic-interference or “EMI” filter) and reduce the PFC circuit's output current ripple (thereby easing the design requirements of its output capacitor).
One drawback of interleaving is the potential for imbalance among its phases, which may be introduced by differences in the performance of devices in each phase (caused by, for example, manufacturing defects). The asymmetry among the phases may also produce undesirable effects, such as overheating of devices that carry the output voltage or current (such as switches and diodes) and saturation of devices (such as inductors in a boost circuit). Thus, balancing of interleaved phases is important for both accuracy of operation and long-term reliability.
One way to balance the phases in an interleaved circuit is to measure an output current or voltage of each phase and, if there is an imbalance, to vary accordingly the length of time that each phase is active. For example, two interleaved phases may be controlled by a clock having a 50% duty cycle (i.e., the clock signal is high half of the time and low half of the time); the first phase may have “stronger” devices that output 1.1 A of current, and the second phase may have “weaker” devices that output 0.9 A of current. The greater current in the first phase may lead to the first-phase devices wearing out sooner than expected and may saturate and/or overheat other components in the system. To compensate, the duty cycle of the first phase may be reduced to 45% and the duty cycle of the second phase increased to 55%, thereby balancing out the energy (i.e., the product of current and time) produced by the two phases.
Existing systems may sense the output current and adjust the duty cycle of a control signal accordingly, but they do so by introducing multiple sensing and control devices that themselves introduce further discrepancies into the circuit. FIG. 1A, for example, illustrates a DC-to-DC boost converter circuit 100 having three interleaved phases 102. The sum of currents through the inductors 104 is sensed through an input-sensing resistor 106, which provides the output current information to the main control loop 108. In order to adjust the duty cycle of each phase, however, the control loop 108 also requires the sensing of the switch currents 110 of each individual phase. Inconsistencies, defects, etc. in the multiple sensing devices may introduce errors in the calibration of the phases 102, resulting in a mis-calibrated circuit 100. Another example is illustrated in FIG. 1B, in which a PFC correction circuit 150 uses two sensing resistors 152 to sense the currents in two interleaved phases 154 for analysis in a current loop controller 156. Like the above example shown in FIG. 1A, the two sensing resistors 152 may introduce errors in the control of the phases 154 if, for example, one resistor is manufactured with a higher- or lower-than-expected resistance.
Thus, a need exists for a way to calibrate the phases in an interleaved circuit, such as a power or PFC circuit, in such a way that does not introduce errors that undermine the calibration, thereby allowing a more precise balancing of the currents (or other electrical parameter, such as voltage) in each phase.